So long as you dont have any leaks, I wouldnt worry about it... The leak thing is kind of old news... I made the switch in a 200Kmi 1983 MB diesel without problems...
I think it wouldnt hurt to use a 10w-30, considering your climate, although a good 5w-30 would be fine as well. You see, when the oil warms up a bit, they will both be the same viscosity anyway.
Since you seem a bit curious that temperature might be an issue, you might consider the German castrol synthetic 0w-30. Its viscosity index makes it a bit thicker, and it meets a higher spec (ACEA A3), that is if you can find it. I wouldnt suggest using the made in the USA syntec 0w-30.
All in all,.Castrol GTX is a fine oil, and their 10w-30 is one of their best products and one of the best dine 10w-30s out on the market. I have had good results with their 5w-30 as well.
FOr mileage, engine cleanliness, drain time extension, etc., you could consider the synthetic.
5 vs 10w-30 is a personal choice, you could try both, do oil analyses on each and see for sure which works better.
Dino vs synth is also a personal choice. Remember, synthetics are the wave of the future, but their performance at low mileage drain intervals isnt anything spectacular compared to dino oils. You have to decide, your vehicle will be fine either way.
JMH